Quadrupole ion trap mass analyzers are widely employed for mass spectrometric analysis of a variety of substances, and are characterized by their high sensitivity and ability to perform multiple stages of isolation and fragmentation, commonly referred to in the art as MSn. In a quadrupole ion trap mass analyzer, ions are confined by oscillatory fields generated by the application of suitable voltages to the ion trap and mass-sequentially ejected to a detector (e.g., by the method of resonance ejection) for acquisition of a mass spectrum. In addition to the electric fields generated by the applied voltages, the ions are also subject to and influenced by electric fields that are generated in the ion trap by the ions themselves. The self-generated electric fields have a characteristic strength that increases with the density of the ion population. The presence of non-trivial self-generated electric fields has a substantial effect on ion behavior, particularly with respect to resonant ejection, which may adversely impact the mass accuracy of the ion peaks detected in the mass spectrum.
In order to avoid or minimize the degradation of performance associated with self-generated electric fields, ion trap mass analyzers are conventionally operated with ion populations for which the self-generated electric fields are substantially smaller than the applied electric fields (i.e., the main trapping and resonant excitation fields). Thus, the maximum density of the ion population is set to a value at which self-generated fields do not appreciably influence ion behavior. Such limits are known as space charge limits.
Operation of ion trap mass analyzers below the space charge limit, while producing acceptable mass accuracy, has the undesirable effect of reducing instrument dynamic range. It may be desirable, particularly when measuring substances present over a large range of concentrations, to fill the ion trap with relatively greater numbers of ions. Filling the trap with greater numbers of ions also increases the ratio of signal to noise, resulting in a more reliable measurement. Thus, there is a need in the mass spectrometry art for a technique to perform mass analysis of a large ion population without sacrificing mass accuracy.